Insulated gate semiconductor devices, such as metal oxide semiconductor field effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs), are widely used as power switching devices. In an insulated gate semiconductor device, a channel is formed in a well region when a voltage equal to or higher than a threshold voltage is applied to a gate electrode, and the insulated gate semiconductor device can be brought into the on state accordingly. The insulated gate semiconductor devices are put to practical use as trench-gate semiconductor devices. In such a trench-gate semiconductor device, a trench is formed so as to extend from the front surface of a semiconductor layer to a drift layer, and a well region on the side surface of the trench is used as a channel. This configuration can improve the density across the channel width, so that the cell pitch can be reduced to improve the performance capabilities of the device.
Semiconductor devices made of silicon carbide (SiC) (hereinafter referred to as “silicon carbide semiconductor devices”) have received attention as the next generation of high-voltage and low-loss semiconductor devices, and development has been proceeding on trench-gate silicon carbide semiconductor devices.
Unfortunately, the electric field is concentrated in the trench bottom when a high voltage is applied to the trench-gate semiconductor device in the off state. In particular, the electric field concentration in the trench bottom of the trench-gate silicon carbide semiconductor device is often a matter of concern because SiC has a high dielectric strength and thus the electric field concentration in the trench bottom is likely to cause a breakdown of a gate insulating film before an avalanche breakdown occurs in the drift layer.
To reduce the electric field concentration in the trench bottom, a configuration has been proposed which includes, in the trench, a protective layer having a conductivity type different from that of the drift layer. For example, in a case where the protective layer is provided in the drift layer below the trench, a depletion layer extends from the protective layer in the off state, and the electric field in the trench bottom can be reduced accordingly (see, for example, Patent Document 1).